In the variable-speed transmission art it is known to provide a hydrostatic transmission which comprises a variable-displacement pump whose input shaft is driven by a prime mover, e.g. an electric motor or an internal-combustion engine, the pump being hydraulically connected to a hydraulic motor whose output shaft drives a load, e.g. a machine or a wheel or a pair of wheels of a vehicle.
In hydrostatic transmissions, the pump and the motor can each be of the axial-piston type, i.e. can be hydrostatic machines having a cylinder drum rotatable about an axis and provided with angularly equispaced cylinder bores in each of which a piston is axially reciprocable.
In such transmissions, moreover, the pump is generally of the swash-plate type. In a swash-plate pump, the input shaft is connected to the cylinder drum and rotates the latter about the axis of the cylinder drum which generally coincides with the axis of the shaft. The pistons react against an inclined disk which can be tilted at selected angles to the axis of the shaft and drum. When, for example, this inclined disk has its planar reaction surface perpendicular to the axis of the drum, the pistons do not undergo any excursion upon rotation of the drum and the pump is said to be in a neutral setting. From this neutral setting, the inclined disk can be tilted to one side or the other to varying degrees, thereby forcing the pistons during part of the rotation of the drum inwardly and drawing the pistons outwardly during another part of the rotation of the drum to alternately express fluid from and draw fluid into the respective cylinders.
The drum can cooperate with a valve plate or a surface provided with a pair of orifices which can serve as discharge and intake orifices depending upon the tilt of the inclined disk. The degree of tilt of the disk from the neutral position, therefore, determines the fluid displacement per revolution of the cylinder drum (stroke volume per revolution) while the direction of tilt determines which of the orifices is to serve as the discharge or pressure orifice and which of the orifices is to serve as the intake or suction orifice.
Manual or hydraulic servomotor means can be provided for tilting the inclinable disk of such a variable-displacement hydrostatic pump.
In such hydrostatic transmissions, moreover, the hydraulic motor may be constituted of the drive-flange type. In this type of construction, the cylinder drum is rotatable about an axis which is generally inclined to the shaft axis and the output shaft is formed with a reaction or driving flange which generally lies perpendicular to the shaft axis. The pistons of the cylinders of the drum drivingly engage the flange so that, when fluid is forced into one of the ports of the hydrostatic pump and is drained from the other, the cylinder is rotated by forcing the pistons successively outwardly to simultaneously drive the output shaft. The piston heads may engage in sockets formed in or attached to the driving flange.
The hydrostatic pump is connected to the hydrostatic motor via passages which generally connect one port of the hydrostatic motor with one port or orifice of the pump and the other port of the hydrostatic motor with the other port or orifice of the pump.
It has been found in practice (see German printed application -- Offenlegungsschrift -- DT-OS No. 1 816 183) that in a hydrostatic transmission in which the pump is of the inclined-disk type and the hydrostatic motor is of the drive-flange type, the advantages of both hydrostatic machine configurations are combined to provide a relatively small, light-weight and inexpensive pump configuration in association with a most effective or efficient hydraulic motor.
With this configuration, however, there have generally been three conditions which have been observed:
(a) the hydraulic motor is a constant-displacement motor, i.e. the motor has an invariable stroke volume per revolution;
(b) the hydraulic motor is provided in a construction in which the cylinder drum is surrounded by a housing whose axis determines the axis of the cylinder drum; and
(c) the transmission has been provided with the hydrostatic machines in spaced or separated relationship, i.e. the hydrostatic pump is usually separated from the hydrostatic motor, the two being connected by hydraulic lines.
In practice, moreover, the configuration of the pump as an inclined-disk pump provides certain advantages as enumerated above, primarily in terms of small volume, which are lost when the hydrostatic motor is of the drive-flange type.
It should also be mentioned that there are hydrostatic transmissions which have been proposed or are in use in compact configuration, i.e. the pump and motor are disposed in a common housing. In this case, both the hydrostatic pump and the hydrostatic motor are provided in the inclined-disk configuration coaxial to one another and the cylinder drums are disposed back to back against a common body which is provided with the passages through which the fluid medium is delivered by the pump to the motor and is returned from the motor to the pump. Such constructions, however, have the disadvantage that the hydrostatic motor operates less efficiently than a motor of the drive-flange type.